The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Processing of acid gases and especially of sulfur containing acid gases is well known in the art. Most typically, sulfur containing acid gases are processed to enrich the gas in hydrogen sulfide, or if concentrations are already sufficient, the sulfur containing acid gases are directly fed into a Claus plant for conversion into elemental sulfur. While the Claus reaction is at least conceptually simple and relatively easy to implement, significant limitations are present where the feed gas that has relatively high concentrations of ammonia. For example, ammonia is found next to hydrogen sulfide in relatively high concentrations in sour water stripper gases, coke vessel off gases, off gases from acid gas removal units, low-temperature gasification off gases, and/or hydroprocessing off gases.
Regardless of the particular source, Claus plants require significant modifications to cope with higher ammonia concentrations (e.g., require a modified Claus burner and/or use of oxygen enrichment for thermal decomposition). On the other hand, dedicated upstream systems may be employed that specifically deal with the excess ammonia. For example, WO2006106289 teaches combustion of ammonia and hydrogen sulfide in a single Claus thermal stage, while U.S. Pat. No. 5,853,682 teaches catalytic ammonia cracking. Unfortunately, most of these systems and methods tend to require significant modification of existing Claus plants and are thus not economically attractive. All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
In other attempts to deal with ammonia and hydrogen sulfide as described for example in U.S. Pat. No. 7,258,848, various systems and methods are presented in which ammonia is contacted with a concentrated acid to produce an ammonium salt solution that is then transferred to a stripper operating at relatively low temperature to remove hydrogen sulfide from therefrom before the so stripped solution is contacted with a base to obtain the corresponding crystalline ammonium salts. The remaining liquid can then be recycled to the absorption step. While such process is fairly effective in separating ammonia from hydrogen sulfide, numerous problems arise. First, due to the concentrated acid solution, precipitation of corresponding ammonium salts may occur in the absorption step, especially where the ammonia concentration is relatively high. Second, due to the stripping step, the separated hydrogen sulfide may be released in a more diluted form that may necessitate a downstream concentration step.
In another known example, US 2006/0272502, ammonia and hydrogen sulfide (and other impurities) are sequentially isolated in a process where ammonia is removed at low temperatures and elevated pressure using an aqueous acidic wash liquid while hydrogen sulfide is removed in a later step via an alkaline washing liquid. Such process not only requires substantial equipment, but also regeneration of multiple solvents, adding significant costs. Moreover, and even more disadvantageous, the processes of the '502 application are limited to a maximum concentration of ammonia of 0.6 vol %.
Therefore, even though numerous systems and methods are known on the art to process ammonia-rich acid gases, various difficulties still remain. Thus, there is still a need for improved Claus plant pre-processing systems and methods for removal of ammonia from Claus plant feed gases.